since Internet Explorer seems to be the only browser vulnerable to
That is the false assumption which is largely responsible of your situation because on February 2015, Microsoft already released HTTP Strict Transport Security in Internet Explorer 11. It is clear, regarding the error message you got, that your IE version is 11 or higher.
i.e a DOS of some sort
No, SSL stripping has nothing to do with DoS.
exception.runtimeerror : request.write called on a request after
request.finish was called
It is normal you got that error since you know the purpose of HSTS and which not not only makes your connection fails, but can also, in case you attempt your MITM on other users, inform them (your potential victims) that they have been tampered by a third party.
And when it does work I get this error message :
Now you can guess you misinterpreted the events: since the error message by itself warns you that the connection failed.
Now, a question you may ask: is this the end of SSLstrip ? Luckily (or unluckily, depending on the colors you wear), there is an other opensource tool that uses an updated version of SSLstrip called mana. If you are running Ubuntu, Kali or any other Debian distro, you can install it by runnin:
apt-get install mana-toolkit
So in addition to HSTS limitations, mana overcomes and tricks HSTS.